Radio circuit for static limitation



UUOH MM HEUUM i Filed Feb. 11, 1935 2 sheets-sheet 2 m T M m u h ll 4))u I". "I W u l? u m I m "r R 3 s \u M H a 9 ZIHI Jw H\ 4 2 I 1 l I-HIi=2 2\ u w lo M l 2 a I v 1 If? m I H |9 9/v w *9 4 -v\ \9 J 5 9 3 4 616 ...w 9 a 9 FIGURE. V

136 177 mas/o AMPLIFIER LOUD s efllrse DETECTOR RflD/O FREQUENC YAMPLIFIER GATE CIRCUIT FIGURE. VI

IN? E R175 0/1775 FREQUENC Y o m 6 1W M wn D R M mm mm L W can: C/RCUITSPEAKER FIRS 7" TECTOR RAP/0 FRIOUf/W Y INVENTOR DAVID C MCCAA BY GQ/PAT T ORNEY Patented Mar. 29, 1938 UNITED STATES Search PATENT OFFICERADIO CIRCUIT FOR STATIC LIMITATION David G. McCaa, Lancaster, Pa.,assignor to Alan N. Mann, Scarsdale, N. Y., as trustee ApplicationFebruary 11, 1935, Serial No. 6,015

9 Claims.

My invention relates to radio receiving circuits. More particularly itrelates to a radio receiving circuit in which thermionic tubes areemployed for limiting static impulses to predetermined values. In myco-pending application, Serial No. 6,014 filed February 11, 1935,entitled improvements in Thermionic tube circuits, I have disclosedspecific circuits which may be employed as voltage limiting devices. Inthis application, I shall describe improvements in the aforesaidcircuits and novel means in which said circuits may be employed in radioreceiving circuits.

One of the objects of my invention lies in improvements in thermionictube circuits for limiting and amplifying voltage impulses. Anotherobject is to incorporate voltage limiting circuits in a radio receivingsystem. A further object is to automatically adjust the voltage gain ofthe re- .ceiving circuits with respect to the incoming carrier currentsso that the gate or limiting action .will be fully and automaticallyutilized. A further object is to provide a voltage limiting system ca-.pable of being extended to sharp, well defined, output limits byexceedingly small input voltages so that the system may be used in thatpart of the receiving system in which the input voltage ratio is amaximum.

In the accompanying drawings: Figure I represents the basic circuit Iemploy.

Figure II shows a family of characteristic grid voltage-plate currentcurves obtained from the circuit of Figure I.

Figure III represents two stages of the arrangement of Figure I.

Figure IV shows the over-all grid voltage-plate current characteristicof the circuit of Figure III.

Figure V illustrates an alternative arrangement to Figure III applied toa radio receiver.

Figure VI is a schematic diagram of a radio receiving circuit embodyingFigure III.

Figure VII is a schematic diagram of a superheterodyne receiver in whichFigure III is employed at intermediate radio frequency and withautomatic gain control.

In Figure I, l represents a screen grid tube in which 2 is the grid, 3the cathode, 4 the heater, 5 the screen grid, and 6 the anode. The inputterminals of the tube and circuit are I and 8. A suitable bias for thegrid 2 may be obtained by adjusting the slider of potentiometer 9 whichis shunted around battery I0. One terminal of the battery connects tocathode 3 which is grounded at II. The slider of 9 is connected to 8which is by-passed to 3 by condenser l2,

The negative terminal of B battery [3 is connected to ground H. Thepositive terminal of I3 is connected to a non-inductive'and relativelyhigh resistance I4 which in turn connects to anode 6. The anode 6 isconnected to an output terminal l5 by condenser 16. The remaining outputterminal I! is connected to ground I I. A potentiometer I8 is shuntedacross all or a suitable portion of B battery I3. The slider ofpotentiometer I8 is connected to screen grid 5. The screen grid may beby-passed to ground by condenser I9 and the B battery may also bebypassed by a condenser 20.

By employing suitable values, by the way of example, tube RCA 224, gridvoltage -1.5, heater voltage 2.7, B battery 22 volts, screen gridvoltage 6, and. a megohm resistance in the external anode circuit, theunusual characteristic of grid voltage plotted against plate currentillustrated in Figure II is obtained. The unusual abrupt change from thesloping character of the curve to the straight horizontal line,represented by angle A, is secured by a novel combination of means. Thehigh external plate resistance and the low B voltage tend to straightenout the characteristic curve and to limit the anode voltages to valueswhich normally insure anode current saturation. The adjustment of thescreen grid voltage is a further insurance of anode saturation currents,and aid in positioning angle A, and means of determining the steepnessof the slope of the grid voltage plate current characteristic curve. Thegrid voltage determines the normal position of the grid on the slope ofthe characteristic curve.

As the grid voltage is adjusted from positive toward negative (seeFigure II) the anode current remains constant, until the angle A isreached. At this point the anode current changes very abruptly and asthe grid is made more negative the curve is at first a straight line.Finally the curve begins to bend and approaches zero anode current, asthe grid is made more and more negative. The theory and operation ofFigure I is more fully described in my co-pending application mentionedabove.

In Figure III two tubes and circuits, similar to the one illustrated inFigure I, are connected in series. A unipotential cathode tube such asan RCA 224 is represented at 2|. In this tube 22 is the control grid, 23the unipotential cathode, 24 the heater, 25 the screen grid, and 26 theanode. The control grid is connected to input terminal 21. The otherinput terminal is 28 which connects to the slider of potentiometer 29.The po- HUGH;

tentiometer 29 is shunted around biasing battery 30. The positiveterminal of 30 and the cathode 23 are both grounded at 3|. A by-passcondenser 32 connects 28 to 23.

The negative terminal of B battery 33 is grounded at 3|. The positiveterminal of 33 conmeets to one terminal of resistance 34. The remainingterminal of 34 connects to the anode 26. A potentiometer 38 shunting allor part of battery 33 may be used to control the potential of screengrid 25 by means of the slider on 38. The screen grid 25 may beconnected to ground 3| by condenser 39. The B battery may be by-passedby condenser 40.

The second stage employs screen grid tube 4|, RCA type 224 by way ofexample. In this tube 42 is the control grid, 43 the unipotentialcathode, 44 the heater, 45 the screen grid, and 46 the anode. Apotentiometer 49 is connected across battery 59 to bias the cathode 43positive with respect to the grid 42. The slider of 49 provides means tosuitably adjust the biasing voltage. The negative terminal of 50 isgrounded at The slider of potentiometer 49 is connected to ground 5| byby-pass condenser 52.

The negative terminal of B battery 53 is grounded at 5|. The positiveterminal of this battery is connected to one end of resistance 54. Theremaining terminal of 54 is. connected to anode 46. The anode 4'6 isconnected to output terminal 55 by condenser 56. The other outputterminal 5'! is grounded at 5|. A potentiometer 58 shunts all or asuitable part of 53. The screen grid 45 is connected to the slider ofthe potentiometer 58. The screen grid may be bypassed to ground bycondenser 59. A by-pass ggndenser 60 may be shunted across B battery Thecharacteristic of each of these separate stages is similar to thatillustrated in Figure II. The values given above for Figure I may beemployed in the circuits of Figure III. When a varying input voltage isapplied between 2'! and 28, the plate or anode current varies inresistance 34 as shown in Figure II. As the anode current in 2|decreases, the anode 26 will become more positive with respect to groundbecause of less voltage drop in 34. Since 42 is directly connected to26, as 26 becomes more positive, 42 will also become more positive withrespect to ground, overcoming the bias voltage 49. The increasingly lessnegative, or even positive voltage on 42 with respect to cathode 43,causes the anode current in 4| to reach saturation represented by angleA as the anode current of 2| is falling.

On the other hand as the anode current of 2| approaches saturation, themaximum voltage drop is approached in resistance 34 and anode 26 becomesless positive or approaches its minimum positive charge. The minimumpositive voltage on anode 26 also means minimum positive voltage oncontrol grid 42 with respect to ground, because they are directlyconnected. Since the bias 49 makes the grid 42 negative with respect tocathode 43, the result is an increasing negative charge on 42 whichresults in decreasing anode current in 4|.

The net result of the changes just described is illustrated by the gridvoltage curve of the first tube plotted against the anode current of thesecond tube as shown in Figure IV. A and B represent the critical anglesof tubes 2| and 4|. The positions Y and Z at which angles A and B occurmay be determined by the constants chosen,

The actual values of voltage may be of the order given above for FigureI.

I do not intend to be limited to the precise values shown because I havefound a wide range of tubes, resistances, and voltages may be used. Bythe way of preference, the unipotential cathode tube is especiallysuited to the circuits of my invention because of the sharp angle Awhich may be obtained by the use of this type of tube. Likewise thetriode tubes may be used, but I prefer the screen grid tubes because thecapacity between the grid and plate of the triode becomes elTective athigh radio frequencies and, unless neutralized, tends to upset the gateor limiting action. This may even be true, to a slight extent, in screengrid tubes, which may then require some neutralization.

If the second tube of Figure III happens to have a characteristic whichcauses it to draw grid current, (which is shown as Is. in Figure II)when operating near the angle B limit, a load is placed on resistance34. This may disturb the characteristic illustrated in Figure IV.Although this is only true of certain tubes, and may be overcome by thechoice of tubes, or the constants of the circuit, I have been able toentirely eliminate the trouble by the use of a coupling tube.

In Figure V, I have shown a complete radio receiver employing my gate orlimiting circuits and in addition I have shown the use of a couplingtube. The coupling tube is inserted between the two stagcs of Figure IIIto avoid the deleterious efiects of grid current in the second tube andmake the first tube independent of the second.

In Figure V, 66 represents an antenna, 61 a primary inductance which isgrounded at 68. The secondary inductance 69 is coupled to the primary61. Variable tuning capacity is connected in parallel to the inductance,through the large capacity of the by-pass condenser 82. The tunedcircuit comprising 69, I0, 82 is connected to the screen grid type 224tube II as follows: One terminal of condenser 18 connects to the controlgrid 12. The remaining terminal of 10 connects to the unipotentialcathode 13. The heater 14 may be energized by batteries or alternatingcurrent. The screen grid is suitably biased by an adjustablepotentiometer connection.

The control grid 12 is biased negatively by adjusting potentiometer 19which is shunted across battery 80. The positive terminal of 80 and thecathode 13 are both grounded at 8|. The by-pass condenser 82 keeps theradio frequency currents from flowing in I9 and 89. The negativeterminal of B battery 83 is grounded at 8|. The positive terminal isconnected to the resistance 84. The remaining terminal of the resistance84 connects to the anode 1'6.

Potentiometer 88 shunts all or part of battery 83 to provide a suitablebias means for screen grid 15. The screen grid is by-passed to ground bycondenser 89. The B battery 83 may be bypassed by condenser 90. Insteadof directly connecting tube H with the succeeding tube, I interposecoupling tube 9| which may be unipotential cathode tube; such as, theRCA 227.

The grid 92 of tube 9| is connected to the anode 16 of the precedingtube The unipotential cathode 93 is heated by 94 which may be energizedby batteries or alternating current. The cathode is grounded throughself-biasing resistances 95 and 96 which produce a normal voltage dropwhich biases 92 negatively with respect to 93, and

l-UVD unuirinil LHLIIU I H2. This prevents the flow of grid current fromgrid 92 to cathode 93.

The self biasing resistance 96 may be by-passed by a suitable capacity91. The resistance 95 is grounded at 98. The B battery 99 is grounded at98 and its positive terminal is connected to the anode I00.

The coupling tube is connected to the limiter tube I I I. This tube maybe an RCA type 224 and is comprised of control grid H2, unipotentialcathode H3, heater H4, screen grid H5, and anode H6. The control grid H2is connected to the junction of resistances 95 and 96. The cathode H3 isconnected to the slider of potentiometer H9 which shunts battery I20.The negative terminal of I20 is grounded at I2I. A by-pass condenser I22connects the cathode to ground.

The negative terminal of B battery I23 is grounded at I2I. The positiveterminal of I23 is connected to resistance I24. The resistance in turnis connected to the anode H6. The anode H6 is coupled to the detectorcircuit by capacity I 26. A potentiometer I28 shunts all or part ofbattery I23. The slider of I28 is connected to the screen grid H5. Aby-pass condenser I29 may be connected between the screen and ground.Likewise I30 may be connected across I23.

The detector circuit may be of any of the circuit arrangements wellknown to the art. Such circuit is represented by the device within thedotted lines I3I. The circuit illustrated in Figure V, insofar as thegate or limiting action of tubes II and II I is concerned, is the sameas Figure III. The coupling tube connections are arranged so as not toaffect the phase relations of tube 1| with respect to I I I.

The bias of grid III is chosen to fix the gate width YZ when bias ongrid 12 is zero, so that desired incoming modulated carrier currentswill vary between Y and Z. The bias on grid 12 is then adjusted to theoperating position corresponding with X of Figure IV. This isillustrated by the curve C. It is apparent that C will be faithfully andefficiently repeated as variations in the anode or plate current shownas D. Voltages, represented by static currents or otherwise, exceedingthe limits YZ are shown. as E. The effect of E on the anode currentappears as F. The dotted line portion of F is cut oif by the gateaction.

In Figure V the incoming signal currents received by the antenna areinduced in the first tuned circuit. Static currents are likewise set upin the first tuned circuit. The static charges are ordinarily of greatamplitude but of short duration. The signal currents, on the other hand,usually are of long duration. When the signal currents are of lowvoltage, compared with the static voltage, the gate will cut off theexcessive amplitude of the static impulses without affecting thesignals. The detector circuit receiving the equalized desired andundesired voltages will integrate each of the two effects. Due to themarked difference in duration the energy involved in the reproduceddesired signals will far outweigh the undesired static impulses.Although the voltage input at the detector is equal, the sound energyratio of the signals to the static may be of the order of ten to one toa listener.

Thus my invention makes it possible to receive signals throughatmospheric disturbances which may render an ordinary receiverpractically useless. Although I have illustrated a relatively simplesystem operating from batteries, it should control.

beiil'lill iwum be understood that my circuits may be energized entirelyfrom rectified and filtered currents. In Figure VI a tuned radiofrequency system I is shown in schematic outline. In this illustrationI3I may be the circuit of Figure III or V. The potentiometers I32 andI33 are similar to 29 and 49 or 19 and H9. A conventional tuned ortunable radio frequency amplifier is shown as I34. The detector is I35,the audio amplifier I36, and the loud speaker I31.

I prefer operating the gate circuit in front of the radio frequencyamplifier because the static signal ratio appears most favorable at thispoint.

7 However, I have had excellent results when the gate follows the radiofrequency amplifier. In the system of Figure VI, it should be understoodthat I32 and I33 are usually adjusted to the particular signal strengthand gate action desired.

The gate action circuits may be applied to superheterodyne radioreceivers. The circuit of Figure III may be applied in front of theinter-- mediate frequency amplifier as shown in Figure VII. The manualoperation of the gate controls may be simplified by employing automaticvolume Such control may be employed to regulate receiver gain in frontof the gate circuit so that all voltage applied to the gate will besubject to the A. V. C. (automatic volume control) action.

In Figure VII the antenna system I38 is coupled in the conventionalmanner to the radio fre-' quency amplifier I39. The first detector ormixing tube I40 is coupled to the heterodyne oscillator MI and an A. V.C. control circuit I48. The resultant currents, now at intermediatefrequency, are fed to the gate circuit I42. This circuit may be that ofFigure III. The currents in the output of the gate circuit are amplifiedby the intermediate frequency amplifier I43. The second detector isrepresented by I44.

The circuit I48 may include any of the forms of A. V. C. Well known tothose skilled in theart. The voltage derived from the rectifier for theA. V. C. is fed by means of conductors I45 to the radio frequencyamplifier I39 and detector I40 to control the gain of the amplifier andfirst detector. The A. V. C. control tends to keep constant thevoltages, representing desired signals, applied to the gate circuit I42.Thus the gate opening will be suitable for all desired signals andexcessive voltage impulses will be limited as shown in Figure IV. Theoutput signals may be amplified by audio amplifier I46 and reproduced byloudspeaker I41.

I have described several species of circuits which have a gate orlimiting action. These circuits have been applied to different types ofradio receivers. It will occur to those skilled in the art that thecircuits of my invention may be varied and employed in differentarrangements.

By way of example, I have found that the A. V. C. action, instead ofbeing applied to the radio frequency amplifier to control its gain, maybe applied to effectively and variably operate the biases of the gatetubes. In this manner the gate is automatically adjusted to the signalinstead of the signal to the gate. Similar modifications are within thescope of my invention.

I claim:

1. In a device of the character described, a

, pair of screen grid thermionic tubes, means in eluding the screen gridin each of said tubes for .limiting the maximum value of their anodecur- .rents, a single thermionic tube for coupling said ]pair of tubesand connections between said plumlonic tubes, anode circuit resistancesfor limiting the maximum value of the anode currents of such tubes so asto insure normally constant maximum currents in their anode circuits, athird single thermionic tube for coupling said pair of tubes andconnections between said pair of tubes and said third tube so arrangedthat the anode currents of one of said pair of tubes are rising when theanode currents in the other of said pair of tubes are falling.

3. A device as described in claim 2, including biasing means fornormally establishing the anode currents in the anode circuits of saidtubes at values less than said constant currents.

4. A radio receiving device comprising a plur ality of radio frequencyresponsive circuits, means for connecting one of said circuits to another of said circuits comprising a pair of thermionic tubes each havingan anode and a cathode and a control electrode, operating connectionsfor said tubes comprising an anode circuit for each of said tubesincluding a resistance, coupling means for coupling the output of thefirst of said tubes to the input of the second of the tubes, a source ofanode potential for said tubes and biasing means for each of saidcontrol electrodes, the values of the operating connections for saidtubes being such as to cause increasing currents to flow in one anodecircuit concurrent with decreasing currents in the other anode circuitand being such as to cause said pair of tubes to have an operating graphin which the input voltage of the first of said tubes plotted againstthe output current of the second of said tubes is represented by aconstant current portion, a changing current portion and a constantcurrent portion, and being such as to cause the junctions of saidportions to be affected by an input voltage of substantially less thanonetenth of a volt.

5. A structure as specified in claim 4, in which each of the said twotubes has a second control electrode and the operating connections forsaid tubes include means for applying potential to such second controlelectrodes.

6. A structure as specified in claim 4 which further includes means forpreventing grid current in the input circuit of the second of said tubesfrom afiecting the output circuit of the first of said tubes.

7. A structure as specified in claim 4, which further includes a thirdtube connecting the output of the first of said pair of tubes to theinput of the other of said pair of tubes so that grid current in thesecond of said pair of tubes does not afiect the first of said tubes.

8. A radio receiving device including several radio frequency responsivecircuits, a pair of thermionic tubes, connections between one of saidcircuits and the input of the first of said thermionic tubes,connections between the output of the second of said tubes and the otherof said circuits, and operating connections for said tubes comprising ananode circuit for each of said tubes including a resistance, means forlimiting the maximum value of the anode currents in each of said tubes,connections between the first of said tubes and the second of said tubesadapted .to limit the anode currents of said second tube to a maximumand minimum value, a source of anode potential for each of said tubesand means for establishing the normal anode current of the second ofsaid tubes substantially midway be- ,tween the maximum and minimumvalues, the

values of the operating connections for said tubes being such as tocause said two tubes to have an operating graph in which the inputvoltage of said first tube plotted against the output current of saidsecond tube is represented by a constant current portion, a changingcurrent portion and a second current portion and being such as to causethe junctions of said portions to be affected by an input voltage ofsubstantially less than one-tenth of 2, volt.

9. In a radio receiving system, a plurality of signal responsivecircuits, means for coupling one of said circuits to another of saidcircuits, said coupling means comprising a pair of screen gridthermionic tubes, and operating connections for said tubes comprisingmeans for adjusting the voltage applied to the screen grid of such tubesnormally to limit the maximum value of the anode current of each of saidtubes, anode circuits for said tubes including resistances, couplingmeans for coupling the output of one of said tubes to the input of theother of the tubes, a source of anode potential for said tubes andbiasing means for the grids of said tubes, the values of said operatingconnections for said tubes being such that the output of the second ofsaid tubes is limited to sharply defined maximum and minimum valuesrepresented by angular changes in .the characteristic curve and beingsuch that the angular change in such characteristic curve will beaffected by changes of input voltage to the first of said tubes ofsubstantially less than one-tenth of a volt.

DAVID G. McCAA.

